1. Field of the Invention
The present invention relates generally to improvements in the art of monitoring and controlling rod-pumped wells and, more particularly, but not by way of limitation, to methods for obtaining and utilizing the dynamic characteristics of a well pumping system.
2. Brief Description of the Prior Art
The pumping of oil wells is most commonly carried out using a pumping system that is comprised of a pump jack that operates a downhole pump. The pump jack has a walking beam that is oscillated in a vertical plane to reciprocate a polish rod that is suspended from one end of the walking beam and the polish rod is connected to the downhole pump via a plurality of sucker rods that extend in a string from the polish rod to the pump. The oscillation of the walking beam is accomplished by an electric motor having an output shaft that is connected through a suitable transmission to a crank that oscillates the walking beam as the motor output shaft turns. The pump jack is counterbalanced against the static load on the polish rod so that the motor can deliver power to the pump jack at a nearly constant rate related to the difference in the dynamic and static loads on the polish rod during the reciprocation of the polish rod.
While pumping systems of this type provide an effective means of pumping a well, such systems are also subject to a number of problems. A sucker rod might break or the pump might become stuck during the pumping of a well or the situation known as pump-off can occur. Pump-off occurs when the formation, or formations, from which liquids are entering the well cannot keep pace with the pump so that the liquid level in the well drops to the level of the downhole pump. Thereafter, if pumping is continued, the delivery of liquids from the well will be reduced with a consequent loss of efficiency of the pumping operation.
Of these problems, the loss of pumping efficiency due to pump-off has been the most difficult to deal with. A number of devices are available to detect a specific malfunction in the operation of a pumping system; for example, the breakage of a sucker rod, and shut down the system when the malfunction occurs. Pump-off on the other hand, has generally been dealt with by the intermittent operation of the pumping system based on measured dynamic characteristics of the system. The common practice is to obtain a plot of the dynamic load on the polish rod as a function of the polish rod location for each direction of travel of the polish rod as it reciprocates in the well and, from the dynamic characteristics, predict a schedule of operation of the pumping system that will prevent pump-off from occurring.
There are several difficulties with this approach. Initially, the dynamic characteristics of a pumping system depend upon factors such as the composition, density, viscosity and temperature of the well liquids and the size of the sucker rod and depth of the well. Some of these factors often change with time. Thus, a pumping schedule based on the dynamic characteristics of a pumping system at one time may very well not be suitable for pumping a well at a later time. Moreover, the intermittent operation approach based on a time schedule for pumping system operation is often inefficient. Maximum efficiency of the operation of a pumping system occurs when the well is nearly pumped-off and no fixed time schedule can be established for a well such that this condition is met during the operation of the pumping system. Rather, as a practical matter, it has been necessary in the past to permit liquids to accumulate for long periods of time and then pump the well down. Finally, equipment used to measure the dynamic characteristics of a pumping system has, in the past, often been expensive.
Expense has also generally precluded attempts to achieve maximum efficiency of pumping operations by controlling a well such that the pumping system operates only under nearly pumped-off conditions. Because of the above-mentioned problems with intermittent, time-determined operation, some work has been directed toward the development of equipment which can detect pump-off and control the operation of the pumping system about pump-off. However, such work has not generally lead to the control of well pumping systems in part because of the cost of equipment involved in measuring the dynamic characteristics of well pumping systems. Also, such lack of development stems partly from the variation of the dynamic characteristics of a pumping system with time. In general, no well control system has been developed prior to the present invention that can economically control the operation of a well pumping system to continuously provide maximum efficiency of operation of the pumping system.